An angular position sensor detects the angular orientation of a rotating shaft with respect to a housing and normally consists of a rotor fitted on the shaft and a housing which surrounds the rotor. Most existing angular position sensors have an annular resistive element that surrounds the rotor and a brush that makes electrical contact with the rotor to measure changes in resistance to determine the angular orientation of the rotor.
Linear position sensors are employed to measure linear movement of one object with respect to another. Most existing linear position sensors include a generally planar resistive member along which moves the distal end of a wiper. For both angular and linear position sensors employing resistance, an electrical circuit is provided to measure the change in resistance that occurs as the wiper moves along the resistive member to thereby determine the position of the wiper with respect to the resistor.
Where a wiper moves along a resistive member, the wiper may cause small particles of material to be loosened from the resistive member and the loosened particles may cause contamination of the surrounding environment in which the detector is positioned. For example, if the detector is used to detect the amount of liquid in a container, the particles loosened by the wiper may contaminate the liquid. Alternately, particles in the surrounding environment in which the detector is located may interfere with the movement of the wiper with respect to the resistor and cause the device to provide an incorrect reading of the position of the wiper. Furthermore, the movement of the wiper across the resistive surface over a long period of time may alter the resistive qualities of the resistive material, or wear the resistive material away altogether such that the device no longer provides an accurate reading of position.
To avoid the problems that occur in which variable position sensors that rely upon contact between a stationary member and a moving member, it is desirable that the parts be made so as not to require such contact. Efforts have been made to use magnetism and magnetic fields to provide a position sensor that does not require physical contact between a stationary part and a moveable part, but such magnetically operated position sensors may not be useable in circumstances where the magnetic field will interfere with other adjacent devices.
Variable capacitors have also been used as position sensors. Such variable capacitors have a stationary plate or electrode and a parallel moveable electrode. The moveable electrode is moveable between a first position in which the two electrodes are overlapping, or aligned with each other so as to maximize capacitance to a second misaligned position in which the electrodes are not overlapping so that capacitance is minimized. Such variable capacitance requires a relatively large amount of space to provide the same degree of accuracy as resistor type position sensor for several reasons. One is that it is difficult to measure small changes in capacitance and another is that enough space must be provided to allow the plates to become totally misaligned from each other. In the case of an angular position sensor, existing variable capacitors are only rotatable through one hundred eighty degrees, wherein many uses require accurate measurement through three hundred sixty degrees of rotation.
It would be desirable, therefore, to provide a variable position sensor that will accurately provide a reading of the position of a first member that is moveable with respect to a second member and does not rely upon physical contact between the members.